Magnetic core current regulating circuit



Q. WTSIMKINS MAGNETIC com: CURRENT REGULATING cmcun April 14, 1.959

Filed May 28; 1956 FIG.

FIG. 2

PULSE SOURCE PULSE SOURCE PULSE SOURCE FIG. 3

FIG. 4

) INVENTOR 6?. W S IMK INS Br UL .JQ- k ATTORNEY United States PatentOfilice Quinton W. Simkins, Chatham Township, Morris County,

N.J., assignor to Bell Telephone Laboratories, Incorporated, New York,N.Y., a corporation of New York Application May 28, 1956, Serial No.587,567

9 Claims. (Cl. 32%7) This invention relates to circuits for regulatingcurrent applied to a load and, more particularly, to circuits forapplying constant current pulses to a varying impedance load.

Constant current sources are frequently required in pulse circuits suchas computer systems and are particularly required in computer circuitsusing magnetic cores.

Priorly, constant current sources have been devised using the nonlinearcharacteristics of a pair of serially connecteddiodes connected inseries opposition between a pulse source and a load. A high' voltagesource is connected intermediate the two diodes by means of a highresistance. This type circuit delivers a current to the pulse sourcethrough one of the diodes and, in response to an incoming pulse, thecurrent formerly delivered through this one diode is cut oil and thiscurrent is now delivered to the load through the other diode. The seriesresistance between the high voltage source and the load limits thecurrent. This limiting action, however, is achieved at the expense ofwasted power and the requirement of a source o'f'v'oltage many timeshigher than that required to be applied across the load.

Magnetically operated load current control devices, commonly calledsaturable core reactors, are well known in the art. They are essentiallya transformer having two or more windings, one of which is seriallyconnected between an alternating current source and a load. Thereactance of this series connected winding is under the control ofanother winding called the control winding. A direct current is fedthrough the control winding to establish a predetermined flux throughthe transformer core and the impedance of the series connected windingis directly proportional to this control current. The impedance of thisseries winding therefore varies linearly with the control windingcurrent. Saturable core reactors, however, exhibit certain limitationswith regard to use as current limiting devices. Because of the extensiveflux paths of Saturable cores, it is difiicult to establish a fluxthrough these paths by means of pulses applied at megacycle frequencies.Further, even if a saturable core reactor were designed for use in themegacycle range it would require a relatively large control current to II achieve current limiting by maintaining the core in its highlysaturated state.

Accordingly, it is an object of this invention to provide a regulatedconstant current from a relatively low voltage source.

It is another object of this invention to provide a i constant currentsource adapted to drive a variable load, which source consumes a minimumamount of power.

It is a further object of this invention to provide a simplifiedconstant current generator which contains few components.

It isa further object of this invention to provide a current source fordriving magnetic core circuits which compensates for changes in the loadimpedance because of changes in the ambient temperature.

It is another object of this invention to provide a 2,882,482 PatentedApr. 14, 1959 2 current source adapted to produce rectangular currentpulses having short rise and fall times.

It is still another object of this invention to provide a generatordelivering current pulses of predetermined amplitude independent of theload impedance.

Briefly, I have discovered that, by serially connecting a winding of asquare loop magnetic core between a voltage source and a load andpreventing the core from completely switching, the core winding presentsa bilinear impedance, which impedance can be employed as a currentlimiter. In accordance with aspects of this invention, a magnetic corehaving a substantially rectangular hysteresis loop has a windingconnected in series with a source of potential and a switch. The switchis adapted to be actuated at a speed which limits the duration of thepulses to a period insufficient completely to reverse the magnetizationof the core even though the magntiude of the current is otherwisesuflicient to do so. The core and its included winding define a currentlimiting device which presents a first very low linear impedance to lowvalues of current but, in response to values exceeding a threshold orminimum value, presents a very high linear impedance. Thus, the corewinding can be said to exhibit a bilinear impedance. While this currentis of insufiicient duration completely to reverse the magnetization ofthe core, the core is partially switched. Continuous current regulationis assuredby resetting the core 'to its initial condition ofmagnetization before the core is completely switched.

It is a feature of this invention to connect a winding of a magneticcore having a substantially rectangular hysteresis loop in series with anonlinear load and to deliver current pulses through the winding to theload, which pulses are of insufiicient duration completely to reversethe magnetziation of themagnetic core. v

It is another feature of this invention serially to connect a voltagesource, a switch, a magnetic core winding and a nonlinear load and toactuate the switch to permit the voltage source to deliver a currentpulse to the load, which pulse is of insuificient duration completely toswitch the core. This current is effectively limited by the bilinearimpedance of the magnetic core winding.

It is another feature of this invention to employ a single magnetic coreas a current regulating device for a plurality of loads by seriallyconnecting these loads to individual pulse sources through individualwindings on the current limiting core.

It is another feature of this invention serially to connect a currentlimiting winding of one magnetic core and a winding of a magnetic coreload, which current limiting winding eifectively changes its thresholdin the same direction and the same magnitude as the load core winding inresponse to changes in ambient temperature and thus compensates for loadthreshold variation caused by changes in ambient temperature.

A complete understanding of this invention and of these and variousother features thereof may be gained from consideration of the followingdetailed description and the accompanying drawing in which:

Fig. l is an idealized graph of the hysteresis curve of a magnetic coreof the type employed with the circuits of Figs. 3 and 4;

Fig. 2 is a plot of the voltage-current characteristics of a constantcurrent source in accordance with this invention;

Fig. 3 depicts, in schematic form, one illustrative embodiment of thisinvention; and

Fig. 4 depicts, in schematic form, another illustrative embodiment ofthis invention.

Referring now to Fig. 1, there is depicted aferromagnetic corehysteresis loop in which the abscissa is the ampere turns of the corewhile the ordinate is the fluX through the core. Point A represents onestate of stable remanent magnetization While point B represents theother stable state of remanent magnetization. Subsequent reference willbe made to this graph in order to explain the operation of the variousembodiments of this invention.

In accordance with aspects of this invention, a series current limitingcircuit includes a pulse voltage source, a winding on a square loopmagnetic core and a load, The pulse current flowing in this seriescircuit is nearly independent of the load impedance due to the switchingaction of the series core. In order to maintain this current regulationthroughout the pulse interval the switching time of the core must bemade longer than the pulse from the voltage source. Assuming arectangular switching waveform for the core, the switching time andvoltage are related to the core properties by the equation 'rzthe timerequired for switching to take place,

N '-the number of turns of the core winding,

I =the total flux to be switched in the core, and

V=.-'-the voltage across the N turn winding.

To insure current regulation throughout the pulse interval, the actualpulse duration 1 must be-such that Since V is equal to the voltage ofthe pulse source minus the load voltage, V will'be a maximum when theload voltage is minimum. From the Equation 2 above, it can be seen thatpartial switching can be obtained throughproper design of the number ofturns on the winding or by controlling the total flux I either bygeometrical design or by proper choice of core material. With referenceto geometrical design, it is advantageous to use toroidal cores and thecross-sectional area of the core is the-most important dimension. Thematerial employed for the core may, for example, be a ferrite or apermalloy tape.

For example, the loadmay include one or more magnetic core windingsserially connected. Normally, these load cores must be completelyswitched during the pulse interval t while the current regulating coreload must not completely switch in order that current limiting takesplace throughout the pulse interval. In order to obtain this diiferen'cein switching times the core material selected for the load core mayadvantageously be one in which switching occurs at alower value ofinduced flux.

The switching time of the load core is linearly related to the ampereturn'drive as'describedby MrKarnaugh in an article of the May 1955Proceedings of the I.R.E., volume 43, No. 5, pages 570 through 583,entitled Pulse Switching Circuits Using Magnetic Cores.

M This relationship is depicted in Fig. 3 of that article. Thentercept'v'and the slop C- are r l ed o he dimensions of the'core andthe magnetic properties of the core material. The current through theload core winding is controlled by the bilinear impedance f the currentregulat-ing core winding. The number of turns of the winding on the loadcore may now be determined to assure complete switching of the load coreduring the pulse interval, which number of turns, in one embodiment,will exceed the number of turns of the current regulating winding.

Fig. 2 depicts a voltage-current characteristic of a con stant currentsource in accordance with this invention in which the current producedthrough the load is proportional to the applied voltage and inverselyproportional to load impedances such that the current is less than apredetermined value designated as I Further, beyond the valuel theincremental impedance of the current source is high so that for allpractical purposes, the pulse current can be considered to be constant.Subsequent reference will be made to this characteristic in explainingthedetailed operation of the circuits of Figs.3 and 4.

Fig. 3 depicts a pulse source 10 connected to the base of transistor 11,the emitter of which is connected to ground. The collector of thistransistor is connected to winding 12 of ferromagnetic core 13. Theother terminal of winding 12 is connected to one terminal of winding 38on core 39 of load 14. The other terminal of winding 38 is connected tosource 15 of negative potential. Also, on core 13 is winding 16 which isconnected to a circuit for resetting the core to its initial state ofmagnetization. This resetting circuit includes source 17 of potential,inductor 18 and variable resistor 19. As shown in Fig. 3, transistor 11is a p-n-p junction transistor. However, an n-p-n transistor may be usedin this circuit reversing the polarity of source 15.

Assume for the purposes of explaining the operation of the circuit ofFig. 3 that the magnetization of core 13 is that depicted by point A inFig. 1. A pulse applied to transistor 11 trom source 10 causestransistor 11 to become conducting. A pulse of current flows from groundthrough the emitter, base and collector of transistor 11 to winding 12of core 13 and finally to source 15. This pulse is of sufficientmagnitude to shift the magnetization of the core from point A on Fig. 1beyond the knee of the curve but of insufiicient duration to switch themagnetization to point B of Fig. 1. Assume for the moment that thispulse is sufficient to magnetize the core at point B of Fig. 1 when thepulse from source 10 is terminated; the transistor 11 becomesnonconducting and the load cur rent ialls to zero. In response to thetermination of load current, the magnetization of core 13 returns topoint C of Fig. 1. The current limiting action of the core winding canbeunderstood from the application of the pulse to the voltage-currentcharacteristic of Fig. 2. Core 13 is partially switched by single pulsesof current flowing through winding 12. Load 14 may advantageously be aWinding of a second ferromagnetic core and the switching characteristicsof the load core are such that the load core is switched while core13-is only partially switched.

While the foregoing explanation of the operation of the circuit of Fig.3 has been made on the assumption that the magnetic core was initiallymagnetized at point A on the hysteresis loop of Fig. 1, it might havebeen maintained at point P or pointv G on the hysteresis loop by meansof a biasing current applied through a biasing wind.- The current hr ughthis i ing in ng Performs a dual function. One of'these functions isthat the value of this current determines the threshold or I pointndieated in Fig. 2 of the current limiting winding as seen from'thepulse source and the other is that of resetting the core to itsquiescentpoint P or G, whichever point was selected after the partialswitching has taken place.

However, some means must be provided for resetting core 13 before it iscompletely switched to point B of Fig. 1. Core 13 niay, however, be ofthe s me geometry and'composi ion as he load core, c mpl e s i chi g ofcore 13 being inhibited .by the combination of a biasing winding and thepreviously mentioned fast acting transistor switch as shown in Fig. 3.Referring to Fig. 2, the dark lines represent the actual impedance ofthe current limiting core winding as seen-by the current pulses throughthe transistor switch. The first line segment has only a slight slopeindicating a very low linear impedance. At the point-designated 1 thesecond line segment begins which has a steep slope indicating a veryhigh linear impedance. V is-the source voltage and the two lightdiagonal lines leaving point V define load lines. The intersection ofthe two load lines and the second im pedance line are illustrativeoperating points. The initial application of voltage causes current toflow until the value I is reached. Beyond this value of current thesharp-slope of the voltage-current characteristic causes the corewinding 12 to present a high linear impedance to the pulse source. Thus,for increases in current beyond 1 only a slight increase in currentreaches load 14 and, sincethe slope of this characteristic as hereindepicted is greatl snag" eratea, the current can be considered to besubstantially constant. 'When transistor 11 becomes nonconducting,because of the removal of the pulse from source 10, the magnetic fieldof winding 13 collapses. The current from source 17 through winding 16now resets core 13 to point P of Fig. 1. Thus the cycle is ready to berepeated.

.From the foregoing explanation, it is apparent that current limitingcan be achieved by connecting a winding of a square loop magnetic corein series between a pulse source and a load and preventing the core frombeing switched. This load may advantageously be another magnetic corewinding which in effect presents a first small impedance prior to itsswitching, a second greater impedance, during switching and a thirdsmaller impedance after switching.

If the flux difierence between points A and B on the hysteresis loop ofFig. 1 is larger than the quantity where E is the pulse voltage acrossthe series winding of the current limiting core, I is the pulseduration, and N is the number of turns of this winding, the core neednot be reset after each pulse applied to the load but may be reset eachsecond, third or some subsequent number of pulses, provided core 13 isnever permitted to switch completely to point B on the hysteresis loopof Fig. 1.

Referring now to Fig. 4, there is depicted a pair of pulse sources 20and 21 employed to gate transistors 34 and 35, respectively. Seriallyconnected between each of the pulse sources and individual loads 22 and23 are windings 26 and 27, respectively. These windings are wound oncommon ferromagnetic core 28. Sources 24 and 25 of negative potentialare connected to respective loads 22 and 23. Transistor 30 is connectedto windings 31 and 32 on core 28 and comprise a blocking oscillatorwhich resets core 28 after every pulse from sources 20 and 21. Thisembodiment illustrates that one square loop magnetic core may beutilized as a current limiting device for two or more circuits. It isnot necessary to reset the core after each pulse. It need only be resetat sutficiently frequent intervals to prevent the core from beingcompletely switched as was previously explained.

The operation of the circuit of Fig. 4 is quite similar to that of Fig.3. Either source 20 or source 21 gates its associated transistor.Assuming that source 20 has gated transistor 34, current flows throughthe transistor emitter, winding 26 and load 22 to source 24. Winding 26effectively presents the previously described bilinear impedance to thispulse such that a constant current is delivered to load 22 and source24. Subsequent to this, pulse source 21 gates transistor 35. In asimilar manner,

' current flows through the emitter of transistor 35, winding 27, load23 and source 25. This current is limited in the same manner as waspreviously explained in con- ;nection with winding 26. In response tothe partial reversal of the flux of core 28, pulses are induced inwinding 32 in a direction to gate transistor 30. The collector oftransistor 30 is connected through winding 31 and resistor 37 to source36 of negative potential. The gating pulse causes transistor 30 tobecome conducting and deliver a pulse through winding 31 resetting themagnetization of core 28 to point A of Fig. 1. A more detailedexplanation of the operation of transistor switches is disclosed inTransactions of A.I.E.E., volume 74, part 1, March 1955, pages 111-121.The use of blocking oscillators utilizing transistors to pulse cores isdisclosed in .A. H. Bobeck application Serial No. 555,976, filedDecember 28, 1955, and an oscillator for resetting a magnetic core isdisclosed in H. E. Vaughan application Serial No. 449,222, filed August11, 1954.

This circuit possesses numerous advantages including compensations forvariations in the load due to changes tageously vary with temperature insuch a manner as to compensate for changes in the magnetic core loadcharacteristic.

. Short rise and fall times of the current pulses occur due to the factthat the current limiting winding presents an initially low value ofimpedance and the only other circuit element controlling the circuit isa transistor switch which rapidly changes from nonconducting toconducting states. The current supplied to the load is therefore aprecisely limited rectangular pulse having short rise and fall times,which characteristics make this circuit particularly adapted to drivesquare loop magnetic core windings.

It is to be understood that the above-described arrangements areillustrative of the application of the principles of the invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed is:

1. An electrical circuit for regulating the current delivered to a loadincluding a first magnetic core having a first winding thereon, saidcircuit comprising a source of pulses, a second magnetic core having asecond winding thereon and means for resetting said second core, saidsecond winding being serially connected between said source and saidfirst winding, said source including switching means and means foractuating said switching means, said first core being adapted to switchin a shorter time than the time required to switch said second core.

2. A current regulating series circuit in accordance with claim 1wherein the geometry of said second core is such that the switching timeof said second core is longer than the switching time of said firstcore.

3. A current regulating series circuit in accordance with claim 1wherein said first winding contains more turns than said second Winding.

4. A current regulating circuit in accordance with claim 1 wherein thecore materials in said first and said second cores are such that asmaller induced flux is required to switch said first core than isrequired to switch said second core.

5. A current regulating series circuit in accordance with claim 1wherein said switching means comprises a transistor having an emitter, acollector and a base, said collector being connected to said secondwinding, said emitter being connected to a source of reference potentialand wherein said switch actuating means includes means for applying apulse to said base.

6. In an electrical device for regulating the current for a plurality ofloads, a magnetic core exhibiting a substantially rectangular hysteresisloop and having a first, a second, a third and a fourth winding thereon,said first, second, and third windings being wound in one direction onsaid core, said fourth winding being wound in an opposite direction onsaid core, a first load connected in series between said first-mentionedwinding and a source of potential, switching means in circuit with saidfirst load, a second load connected in series between said secondwinding and a source of potential, second switching means in circuitwith said second load, means including said first and said secondswitching means for permitting partial switching of said core whilepreventing complete switching of said core, and means for resetting saidcore.

7.In an electrical device in accordance with claim 6 wherein saidresetting means includes a transistor having a collector and a base andfurther includes a source of potential, said collector being connectedthrough said fourth winding to said source of potential, said base beingconnected through said third winding to a source of reference potential,said emitter being connected to a source of reference potential.

8. In an electrical device in accordance with claim 6 wherein said loadsinclude magnetic cores exhibiting rectangular hysteresis loops andfurther include windings on said last-mentioned cores.

9. An electrical circuit for regulating the magnetic switching of afirst magnetic core exhibiting substantially rectangular hysteresischaracteristic having a temperature variant coercive threshold andhaving a winding inductively coupled thereto, a source of pulses, asecond magnetic core having a winding thereon and having a substantiallycorresponding temperature variant coercive threshold as said firstmagnetic core, and circuit means for serially interconnecting saidwinding on said second magnetic core with said source and said windingon said first magnetic core, said first and second magnetic cores beingstructurally dissimilar whereby said first core completely switches itsstate of magnetization while said second core only partially switchesand said pulse source limiting the duration of said pulses to effectuatecomplete switching of said first core but only partial switching ofsaidsecond core.

References Cited in the file of this patent UNITED STATES PATENTS2,719,773 Karnaugh Oct. 4, 1955 2,747,109 Montner May 22 1956 2,770,734Reek Nov. 13, 1956 2,772,357 Wang Nov. '27, 1956

